The Lighthouse Twins trace their passion for seaside sentries to a camping trip they took with their father 48 years ago, when they were 12. They hated camping.

One of the things they hated was how their father made them turn off the flashlights as soon as they crawled into their sleeping bags. He wanted to save the batteries. They wanted to watch out for spiders.

But this particular trip took them from their home in Ohio to Cape Hatteras, in North Carolina, where they camped on the sand — and looked up at the nation’s tallest lighthouse.

Her identical twin, Karen Scanlon, felt a jolt, too, the lighthouse exciting in her a curiosity about maritime history and the shipwrecks such structures are supposed to prevent.

Fast-forward to today — past attending college together in New Mexico, past moving to San Diego with their husbands and raising children here — and this is where those camping-trip stirrings have brought the twins: up the winding stairs of the Old Point Loma Lighthouse and up a ladder into the tower, to a place few are ever allowed and to a task no others are permitted to perform. They get to polish the fragile, 5-foot-tall glass lens inside.

On a recent weekday morning, they rubbed the ridges with tongue depressors wrapped in their grandmother’s faded linen tea towels. They squeegeed the lantern-room windows with a cleaning solution of 6 cups of alcohol, 4 cups of distilled water and four drops of Wisk.

They dusted vents and wiped grime off black metal railings and got down on their khaki-covered knees to vacuum the floor. On a good day, working together, they’re done in four sweaty hours.

Every six weeks, they come back and do it again. All this for a lighthouse that hasn’t been operational since 1891.

But if you think this is drudgery to them, if you think it’s just wrong that they don’t have much time to enjoy the tower’s 360-degree view, think again.

“To us, this is a privilege,” Scanlon said.

It’s a privilege earned. The sisters know more about local lighthouses, this one in particular, than probably anyone. They’ve traced the history, collected old photographs, tracked down and interviewed the children of long-ago lighthouse keepers.

You could say they’ve written the book on the subject. “Lighthouses of San Diego” was published two years ago.

“Oh, we’re just lighthouse dorks,” Fahlen said.

Dorky enough, anyway, to plan vacations based on which lighthouses are nearby. “Why else would you go anywhere?” Fahlen asked.

She spent October on Bodie Island in North Carolina, helping to restore a lighthouse there. In February, she’s going to Scotland for the 200th anniversary of the Bell Rock Lighthouse, popular enough to have its own calendar. (Fahlen took two of the pictures in the 2011 edition.)

Ask them why they do this and they’ll mention the history and the romance and the lore. But mostly it boils down to a question Scanlon asks: “Who doesn’t like lighthouses?”

Historians believe the first lighthouse was built on the island of Pharos in Alexandria, Egypt, in about 280 B.C. It stood some 400 feet tall, and legend says its light — flame and mirrors — could be seen for dozens of miles. It was the last of the Seven Wonders of the Ancient World.

America’s first lighthouse went up in Boston Harbor in 1716 and was burned down during the Revolutionary War. Rebuilt in 1783, it still stands.

That it still stands is a big reason lighthouses are fascinating. Nobody builds working lighthouses anymore; the Global Positioning System renders them largely obsolete. But people still like the idea of something keeping watch, a lone sentinel strong against the elements. They’re comforted that these buildings have outlived earlier generations and might outlive more. Eternity beckons.

Lighthouses also speak to humanity’s better natures — man helping man. They’ve been the subject of poems by Henry Wadsworth Longfellow and songs by James Taylor, used as metaphors for both loneliness and courage. The Internet is awash in lighthouse websites.

And then there’s this: Since early June, a man in Ferndale has intermittently chained himself to a lighthouse replica at the Humboldt County Fairgrounds, protesting plans by the Coast Guard to remove its 150-year-old lens for safekeeping.

The Lighthouse Twins understand that kind of passion.

They were born, they kid you not, at Twin City Hospital in Dennison, Ohio — reportedly the first twins ever born there. Scanlon came out first, six minutes before Fahlen, who is quick to argue, “That doesn’t mean she’s more mature than I am.”

They sometimes wore matching outfits as kids, and they wear matching outfits now as volunteers at the Cabrillo National Monument, where the Old Point Loma Lighthouse stands.

When they clean the multipaneled lighthouse lens, which is hollow, one works outside, the other inside, and the reflections are dizzying, like a funhouse mirror. Twins everywhere.

Although both were bitten by the lighthouse bug during the camping trip all those years ago in Cape Hatteras, their fevers didn’t blossom at the same time. Fahlen was first.

She became intrigued at the optics involved, the glass positioned just so to send light miles out to sea. She used to photograph restoration projects; now she does some of the work herself. She used to write journal articles about lighthouses; now she also edits the work of others.

Her e-mail address has the words “hyper-radial” in it. A hyper-radial is a really big lighthouse lens.

Fahlen jokes that it took her sister awhile “to come out of the lighthouse closet” and admit her fascination straight up. That was about a dozen years ago. Scanlon’s niche as a researcher and writer had been maritime history, which she traces in part to the first book she ever cared about as a child. It was a picture book about the Titanic.

Now the two get together and talk in lighthouse shorthand, Fresnel this and first-order that. To them, Robert Louis Stevenson isn’t the celebrated author of “Treasure Island” but the grandson of Scotland’s most famous lighthouse builder.

They admit that their family and friends don’t always understand why they do what they do. Fahlen said her former husband, bowing out of a trip to tour lighthouses, once told her, “You just see something I don’t see.”

But their children — they each have two — will sometimes call from the road while on vacation. “Lighthouse sighting!” they’ll report. “Lighthouse sighting!”